Known in the art are liquid crystalline trans-2-[4-(4'-cyanophenyloxycarbonyl)phenyl]-5-alkyl-1,3-dioxanes of the formula ##STR3## wherein R is an alkyl radical of a normal structure with 6 or 7 carbon atoms (J. fur prakt. Chem., Bd. 323, No. 6, 1981, Leipzig, H. M. Vorbrodt, S. Deresch, H. Kresse, A. Wiegeleben, D. Denues, H. Laschke, "Synthese and Eigenschaften substituierter 1,3-Dioxane", S. 902-13).
These compounds are characterized by high temperatures of nematic phase formation (135.degree. C.) and by a liability to isomerization to corresponding cis-isomers, whereas liquid crystal materials based on these compounds are inadequate in terms of their electrooptical parameters and cannot be used in the range of negative temperatures.
Also known in the art are 2-(4-substituted phenyl)-5-alkyl-1,3,2-dioxaborinanes of the formula ##STR4## wherein R is an alkyl radical of a normal structure, comprising from 3 to 6 carbon atoms; R' is --OR, --CN, ##STR5## (JP, A, NOS. 61-83190, 61-109792).
These compounds have a rather high temperature of nematic phase formation (not lower than 105.degree. C.) and a low positive anisotropy of permittivity. As a result, the threshold voltage and saturation voltage of the "twist"-effect of the liquid crystal material comprising these compounds are high, and the material becomes inapplicable at low temperatures.